Thermal transfer imaging systems have been used in recent years to make high quality color hard copy from electronic data. These systems basically utilize a recording method in which a donor sheet, having a colorant (i.e., dye or pigment) layer thereon, and a receptor sheet are brought into contact and the donor sheet is heated with a thermal printhead or a laser heat source, for example. By selectively heating and transferring an image forming material from the donor sheet, a desired image is formed on the receptor sheet. This imaging process can involve either mass transfer or dye transfer.
In thermal mass transfer systems, the image is formed by the transfer of a mass of material containing colorant therein, such as pigment-filled polymer coatings, from the donor sheet to the receptor sheet. In thermal dye transfer systems, the image is formed by the transfer of a colorant (the dye) from the donor sheet to the receptor sheet in a controlled fashion (e.g., by sublimation, melt transfer, or diffusion).
A lithographic printing plate comprises a substrate, a hydrophilic surface on the substrate, and a hydrophobic layer disposed in imagewise fashion on the hydrophilic surface. In a traditional printing plate, a hydrophobic photopolymer layer is pre-coated over the entire hydrophilic surface of the substrate. The photopolymer layer is contacted with a mask, exposed, and developed with a liquid developer to remove a portion of the photopolymer layer and form the desired image. The whole process for making such a conventional printing plate is very expensive because it requires masking and it is labor-intensive. Also, it is messy because of the use of liquids, and environmentally unsafe if high pH or organic solvent-based developers are used.
In the past, people have tried with limited success to develop totally dry direct printing plate systems that could solve the above problems. One more economical approach has been to prepare printing plates using thermal mass transfer processes. Thermoplastic resins have generally been used as hydrophobic materials in such thermal mass transfer processes. Uncrosslinked thermoplastic resins are not very resistant to heat or abrasion, however, and printing plates made of this type of resin suffer from short press life due to this poor abrasion resistance. The laser induced ablative transfer of thermally crosslinkable materials to produce a printing plate was disclosed in U.S. Pat. No. 3,964,389. This system reportedly resulted in a printing plate with improved press life. However, this system requires thermal post-curing, which takes considerably more time than a photopolymerizable system.